The present invention relates to a CDMA reception method and CDMA receiver which are used for a mobile communication system based on the CDMA (Coded Division Multiple Access) scheme.
In general, in a mobile communication system based on the CDMA scheme, a signal to be transmitted is spread and transmitted by using a predetermined spreading code on the transmitter side, and the signal received on the receiver side is de-spread by using the spreading code used on the transmitter side to obtain a desired signal. This system also uses the RAKE reception scheme of de-spreading signals received through a plurality of paths (propagation paths) by using reception delay amounts (reception timings) corresponding to the respective paths, detecting the signals, and synthesizing the resultant signals. With this scheme, the influences of multipath interference are suppressed, and a reduction in transmission power is attained.
Conventionally, a CDMA receiver such as a portable telephone used in a mobile communication system based on the CDMA scheme has an arrangement like the one shown in FIGS. 14A and 14B.
As shown in FIG. 14A, the CDMA receiver is comprised of a plurality of FINGER processing sections 2A to 2N respectively having de-spreading sections 8 and channel estimating sections 9 and arranged in parallel with respect to a reception signal 1, a RAKE synthesizing section 3 for synthesizing detection outputs 7 from the FINGER processing sections 2A to 2N, a decoding section 4 for decoding the synthetic output from the RAKE synthesizing section 3, and a synchronous acquisition following section 5 for respectively outputting delay amount designation signals to the FINGER processing sections 2A to 2N on the basis of the reception signal 1.
The reception signal 1 is input to each of the FINGER processing sections 2A to 2N. Each signal is then de-spread by being multiplied by the spreading code used on the transmission side by a correlator 81 of the de-spreading section 8 on the basis of a corresponding delay amount designation signal 6 designated by the synchronous acquisition following section 5, as shown in FIG. 14B.
The synchronous acquisition following section 5 obtains a correlation profile by multiplying the reception signal 1 by the spreading code used on the transmission side while shifting the phase of the code little by little. In the obtained correlation profile, correlation values larger than a predetermined threshold are selected by a predetermined number in the descending order of peak values. In this case, the respective reception delay amounts used to obtain the selected correlation values are sent as the delay amount designation signals 6 optimal for the respective paths to the de-spreading sections 8 of the FINGER processing sections 2A to 2N.
Correlation values 14 obtained by the respective de-spreading sections 8 are input to detection sections 13 of the channel estimating sections 9 and subjected to interpolation of a fading phase shift on the basis of the channel estimation vectors obtained from the correlation values 14 by estimating sections 12. The resultant data are output as detection data 7.
The detection data 7 obtained by the FINGER processing sections 2A to 2N using the reception delay amounts corresponding to the respective paths are added/synthesized by the RAKE synthesizing section 3. The decoding section 4 decodes the resultant data to obtain a desired signal.
In such a conventional CDMA reception method, however, it takes a certain period of time to perform reception delay amount selection processing in the synchronous acquisition following section 5. For this reason, if the reception quality of a predetermined path deteriorates within this processing period, the normal detection data 7 cannot be obtained from the FINGER processing section corresponding to the path. As a consequence, the reception quality of the signal obtained by RAKE synthesis also deteriorates.
It is an object of the present invention to provide a CDMA reception method and CDMA receiver which can suppress a deterioration in the reception quality of a signal obtained by RAKE synthesis even if a reception condition changes within a reception delay amount selection processing period.
In order to achieve the above object, according to the present invention, there is provided a CDMA reception method comprising the steps of presetting a plurality of different reference reception delay amounts on the basis of a correlation profile obtained by de-spreading a reception signal, obtaining a plurality of correlation values on the basis of at least each of the preset reference reception delay amounts, measuring a plurality of interference wave levels on the basis of the obtained correlation values, determining, on the basis of reception qualities obtained from the correlation values corresponding to the measured interference wave levels, whether detection signals obtained from the respective correlation values are synthesized; and synthesizing permitted detection signals and decoding the synthetic signal.